The spine is a flexible column formed of a plurality of bones called vertebrae separated by cartilaginous disks. The spine includes seven cervical (neck) vertebrae, 12 thoracic (chest) vertebrae, five lumbar (lower back) vertebrae, and the fused vertebrae in the sacrum and coccyx that help to form the hip region. While the shapes of individual vertebrae differ among these regions, each is essentially a short hollow tube containing the bundle of nerves known as the spinal cord. Individual nerves, such as those carrying messages to the arms or legs, enter and exit the spinal cord through gaps between vertebrae. The different vertebrae are connected to one another by means of articular processes and intervertebral, fibrocartilaginous bodies.
The spinal disks act as shock absorbers, cushioning the spine, and preventing individual bones from contacting each other. Disks also help to hold the vertebrae together. The weight of the upper body is transferred through the spine to the hips and the legs. The spine is held upright through the work of the back muscles, which are attached to the vertebrae. While the normal spine has no side-to-side curve, it does have a series of front-to-back curves, giving it a gentle “S” shape. The spine curves in at the lumbar region, back out at the thoracic region, and back in at the cervical region.
Various spinal disorders may cause the spine to become misaligned, curved, and/or twisted or result in fractured and/or compressed vertebrae. It is often necessary to surgically correct these spinal disorders. Spinal fusion is a procedure that promotes fusing or growing together of two or more vertebrae in the spine. Spinal fusion can be performed to straighten a spine deformed by scoliosis, neuromuscular disease, cerebral palsy, or other disorders; prevent further deformation; support a spine weakened by infection or tumor; reduce or prevent pain from pinched or injured nerves; or compensate for injured vertebrae or disks. The procedure may improve posture, increase ability to ventilate the lungs, prevent pain, or treat spinal instability and reduce the risk of nerve damage.
One of the types of spinal fusion procedures is a posterior spinal fusion surgery. This procedure is performed posteriorly, or from the back of patient, as opposed to anteriorly, or through the abdomen. Generally the correct curvature is obtained by manipulating the vertebrae into their proper position and securing that position with a rigid system of screws and rods. The screws may be inserted into the pedicles of the vertebrae to act as bone anchors, and the rods may be inserted into heads of the screws. Two rods may run substantially parallel to the spine and secure the spine in the desired shape and curvature. Thus the rods, which are shaped to mimic the correct spinal curvature, force the spine into proper alignment. Bone grafts are then placed between the vertebrae to aid in fusion of the individual vertebrae together to form a correctly aligned spine.
There have been various approaches and systems for performing posterior spinal surgery. Some conventional systems include titanium construction that is compatible with current CT and MRI scanning technology, low profile implant systems, top-loading and top-tightening systems, and other parameters. Some systems also include cross-connectors that allow a one-piece implant to be applied to a dual-rod construct for a top-loading approach.
In minimally invasive surgery (MIS), access to the screws may be severely limited to the space provided by extenders or extension members attached to the screw heads. For example, an exemplary posterior system including screws and extenders may be found in commonly assigned U.S. Application Pub. No. 2010/0036443, entitled “Systems and Methods for Spinal Fixation” filed on Aug. 11, 2009 to Hutton et al. The system may be used in a MIS setting. The extenders may attach to the screw heads at a distal end and extend outside the MIS surgical opening at a proximal end. The extenders are generally tubular and may also include slots along their lengths for insertion of fixation rods. The extenders include a cannula extending from the proximal end to the distal end for insertion of various instruments and setscrews. For example, one or more instruments may be required to position the fixation rods into the heads of the screws, seat the rods within the heads, insert set screws to retain the rods within the heads, and tighten the set screws to rigidly fix the system together to ensure proper bone fusion.
A typical rod reduction instrument may include a cannulated tube that slides over the screw extender. For example, the reduction instrument may include a cannula through which the screw extender may slide. Thus, the reduction instrument may include a larger footprint than the surgical opening. The larger footprint may increase the size of the surgical opening. The reduction instrument may also inhibit loading and insertion of a setscrew used to secure the rod within the screw head. For example, after reduction of the rod into the screw head, a separate instrument may be used to insert the setscrew and advance the setscrew into the screw head to fix the rod therein.